Gabriel decided to execute his plan at night and also when he was going to execute the theft... He decided that it was better to do it tonight itself!
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Eliana located Garrick, who was one of the best students in the House of Light and also the student she placed most focus on.
Other than being a good student, he also had another identity. He was the heir of the Raini Family, which was one of the Major Families in the city. One of the Head Priests in the Church of Light was from the Raini Family as well, and so was Yann.
If she sent anyone else to bring Yann back, she couldn't trust them not to tell everything that happened in the Academy under the pressure of the Raini Family.
She also couldn't send a teacher to bring Yann back suddenly as that could also raise suspicions. At any cost, she didn't want the Raini Family to find out what happened here. Because if they did, the Church was sure to get involved.